The OSI model assigns the functions of path selection and logical addressing
to the OSI network layer (Layer 3). Path selection includes the process of
learning all the paths, or routes, in a network and then forwarding packets
based on those paths or routes. Often the terms path selection and
routing are used interchangeably. In most Cisco documentation and in this
book, routing is the more popular term.

In this chapter, you will learn about the core concepts behind OSI Layer 3.
Because CCNA focuses on TCP/IP, you also will learn about the main Layer 3
protocol used by TCP/IPnamely, the Internet Protocol (IP). This coverage
includes IP addressing, IP routing, and some protocols useful to IP's
effort to deliver packets end to end through a network.

"Do I Know This Already?" Quiz

The purpose of the "Do I Know This Already?" quiz is to help you
decide whether you really need to read the entire chapter. If you already intend
to read the entire chapter, you do not necessarily need to answer these
questions now.

The 12-question quiz, derived from the major sections in the "Foundation
Topics" portion of the chapter, helps you determine how to spend your
limited study time.

Table 5-1 outlines the major topics discussed in this chapter and the
"Do I Know This Already?" quiz questions that correspond to those
topics.

The goal of self-assessment is to gauge your mastery of the topics in this
chapter. If you do not know the answer to a question or are only partially sure
of the answer, you should mark this question wrong for purposes of the
self-assessment. Giving yourself credit for an answer that you correctly guess
skews your self-assessment results and might provide you with a false sense of
security.

Which of the following describes the functions of OSI Layer 3 protocols?

Logical addressing

Physical addressing

Path selection

Arbitration

Error recovery

Imagine that PC1 needs to send some data to PC2, and PC1 and PC2 are
separated by several routers. What are the largest entities that make it from
PC1 to PC2?

Frame

Segment

Packet

L5PDU

L3PDU

L1PDU

Which of the following does a router normally use when making a decision
about routing TCP/IP?

Destination MAC address

Source MAC address

Destination IP address

Source IP address

Destination MAC and IP address

Imagine a network with two routers that are connected with a
point-to-point HDLC serial link. Each router has an Ethernet, with PC1 sharing
the Ethernet with Router1, and PC2 sharing an Ethernet with Router2. When PC1
sends data to PC2, which of the following is true?

Router1 strips the Ethernet header and trailer off the frame received
from PC1, never to be used again.

Router1 encapsulates the Ethernet frame inside an HDLC header and sends
the frame to Router2, which extracts the Ethernet frame for forwarding to
PC2.

Router1 strips the Ethernet header and trailer off the frame received
from PC1, which is exactly re-created by R2 before forwarding data to
PC2.

Router1 removes the Ethernet, IP, and TCP headers, and rebuilds the
appropriate headers before forwarding the packet to Router2.

Which of the following are valid Class C IP addresses?

1.1.1.1

200.1.1.1

128.128.128.128

224.1.1.1

223.223.223.255

What is the range for the values of the first octet for Class A IP
networks?

0 to 127

0 to 126

1 to 127

1 to 126

128 to 191

128 to 192

PC1 and PC2 are on two different Ethernets that are separated by an IP
router. PC1's IP address is 10.1.1.1, and no subnetting is used. Which of
the following addresses could be used for PC2?

10.1.1.2

10.2.2.2

10.200.200.1

9.1.1.1

225.1.1.1

1.1.1.1

How many valid host IP addresses does each Class B network contain?

16,777,214

16,777,216

65,536

65,534

65,532

32,768

32,766

32,764

How many valid host IP addresses does each Class C network contain?

65,536

65,534

65,532

32,768

32,766

256

254

Which of the following protocols allows a client PC to discover the IP
address of another computer, based on that other computer's name?

ARP

RARP

DNS

DHCP

BOOTP

Which of the following protocols allow a client PC to request assignment
of an IP address as well as learn its default gateway?

ARP

RARP

DNS

DHCP

Which term is defined by the following phrase: "the type of
protocol that is being forwarded when routers perform routing."

Routed protocol

Routing protocol

RIP

IOS

Route protocol

The answers to the "Do I Know This Already?" quiz are found in
Appendix A, "Answers to the 'Do I Know This Already?' Quizzes and
Q&A Sections." The suggested choices for your next step are as
follows:

10 or less overall scoreRead the entire chapter. This
includes the "Foundation Topics" and "Foundation Summary"
sections and the "Q&A" section.

11 or 12 overall scoreIf you want more review on these
topics, skip to the "Foundation Summary" section and then go to the
"Q&A" section. Otherwise, move to the next chapter.

Foundation Topics

OSI Layer 3equivalent protocols use routing and
addressing to accomplish their goals. The choices made by the people who
made up addressing greatly affect how routing works, so the two topics are best
described together.

This chapter begins with an overview of the functions of routing and network
layer logical addressing. Following that, the text moves on to the basics of IP
addressing, relating IP addressing to the OSI routing and addressing concepts
covered in the first section. The chapter ends with an introduction to IP
routing protocols.